The present invention relates to a method of reducing the intensity distortion induced by cross phase modulation in a wavelength division multiplexed (WDM) optical fiber transmission system.
An important factor in satisfying ever-increasing requirements concerning data transmission rates lies in mastering the technology of wavelength division multiplexed optical fiber transmission.
Wavelength division multiplexing, referred to below by the initials WDM, consists in combining a plurality of modulated channels in a single fiber, each channel using a different carrier wavelength. Thus, the total data rate of a transmission line is equal to the sum of the data rates of the various channels.
Nevertheless, WDM is subject to propagation limits. These limits are due to linear effects such as group velocity dispersion (GVD), and to non-linear effects such as phase self-modulation and cross phase modulation.
More particularly, the present invention relates to reducing the effect of cross phase modulation (XPM) in WDM optical fiber transmission systems.
XPM is a multichannel effect in which phase modulation is induced in a channel by the intensity of the signal(s) in the adjacent channel(s). This phenomenon thus leads to GVD distortions in the intensity of the signal to be transmitted, thereby limiting the maximum power that can be introduced into a transmission optical fiber.
Unfortunately, the higher the power introduced into a transmission optical fiber, the greater the distance that can be allowed between repeaters in a line, or the greater the total possible transmission distance.
From the article by R. A. Saunders et al., entitled xe2x80x9cCompensation of cross phase modulation in 10 Gbit/s WDM systems by interfering channel prechirpxe2x80x9d, OFC""98, Paper FC3, San Diego, Calif., February 1998, a method of reducing XPM is known in which XPM is compensated by modulating the optical signal for transmission at the emitter with phase opposite to that induced by the XPM.
However, that solution is suitable only for optical fibers that present dispersion that is very low. Furthermore, that method does not take account of the interaction between chromatic dispersion and the Kerr effect.
Furthermore, that method is difficult to implement since it requires full knowledge of all of the channels, e.g. concerning their powers and modulation formats.
The present invention seeks to mitigate those various drawbacks by proposing a method which makes it possible in a manner that is effective and simple to reduce the intensity distortion induced by cross phase modulation in a WDM optical fiber transmission system.
To this end, the invention provides a method of reducing the intensity distortion induced by cross phase modulation in a wavelength division multiplexed optical fiber transmission system comprising a transmission line made up of a plurality of optical fiber segments with repeaters interposed between successive optical fiber segments, the transmission system having N different wavelength channels, where N is an integer greater than unity, wherein, in each repeater interconnecting first and second consecutive fiber segments, a time offset is introduced between the channels in such a manner that compared with the inlet of the first optical segment, the (n+1)th andthe nth channels are offset by xcfx84n at the inlet to the second fiber segment, where n is an integer less than or equal to N, where xcfx84n is selected to be greater than zero and less than a value that eliminates correlation between the intensity distortion contributions of each fiber segment.
The method of the invention may further comprise one or more of the following characteristics:
for all channels xcfx84n=xcfx84;
the time offset xcfx84 is less than about 600 ps;
the time offset xcfx84 is selected in such a manner as to obtain destructive interference between the various contributions to distortion of the individual optical fiber segments;
the time offset xcfx84 lies approximately in the range T/10 to 1.5*T where T is the duration of one bit;
the time offset xcfx84 lies approximately in the range 5 ps to 200 ps; and
it further includes a step of demultiplexing prior to introducing time offsets between the channels, and a step of multiplexing performed after the time offsets between the channels have been introduced.
The invention also provides a unit for implementing the above-defined method, the unit comprising:
an inlet light waveguide for wavelength division multiplexed optical signals;
an outlet light waveguide for wavelength division multiplexed optical signals; and
an optical circulator comprising first, second, and third ports with the first and third ports connected respectively to said inlet light waveguide and to said outlet light waveguide, and with a third light waveguide connected to the second port and comprising a fiber having a Bragg grating associated with each of the N channels to reflect the optical signals of the associated channel back to the second port, the distance In between the nth and (n+1)th Bragg gratings being selected in such a manner as to introduce a total delay xcfx84nxe2x80x3 between the (n+1)th and the nth channel so as to obtain a time offset of xcfx84n between the (n+1)th and the nth channel at the inlet of the fiber segment following said unit relative to the offset between them at the inlet to the optical fiber segment preceding said unit, where xcfx81n is selected to be greater than zero and less than a value that eliminates correlation between the intensity distortion contributions from each fiber segment.
The invention also provides a unit for implementing the above-defined method, the unit comprising:
an inlet light waveguide for wavelength division multiplexed optical signals;
a demultiplexer for demultiplexing wavelength division multiplexed optical signals at the outlet from the inlet light waveguide into N individual channels;
a multiplexer for multiplexing the optical signals at the outlet from the N individual channels, the outlet from the multiplexer being connected to an outlet light waveguide; and
a delay line disposed in each associated individual channel, the length of each delay line being selected in such a manner as to introduce a total delay xcfx84n xe2x80x3 between the (n+1)th and the nth channels so as to obtain between the (n+1)th and the nth channels a time offset of xcfx84n at the inlet to the fiber segment following said unit relative to the offset between them at the inlet to the optical fiber segment preceding said unit, where xcfx84n is selected to be greater than zero and less than a value which eliminates the correlation between the intensity distortion contributions from each fiber segment.